1. Field of the Invention
The invention relates generally to a system for measuring turbidity of liquids, and more particularly to a system including a chamber for receiving a sample liquid, a cover for covering the chamber; and a sensor unit with its bottom immersed in the sample liquid.
2. Description of Related Arts
Turbidity sensors measure suspended matters in water that interfere with the passage of light through the water or in which visual depth is restricted. The turbidity may be caused by a wide variety of suspended materials, such as clay, silt, finely divided organic and inorganic matter, soluble colored organic compounds, plankton and other microscopic organisms and similar substances. Turbidity in water has public health implications due to the possibilities of pathogenic bacteria encased in the particles and thus escaping disinfection processes. Excessive amounts of turbidity also make water aesthetically objectionable. Turbidity of water is very important for evaluating the efficiency of the water treatment and water cleaning processes. The measurement unit for is nephelometric turbidity unit (NTU). An instrument called a nephelometer measures turbidity by measuring the amount of light scattered at an angle. The instrument is calibrated using samples of a standard solution such as formazin, a synthetic polymer. With a standardized procedure for preparing the stock solution of formazin with turbidity 4000 NTU, all other standards with different NTUs can be prepared by proportionally diluting the stock solution. Drinking water should not have turbidity above 1 NTU, although values up to 5 NTU are usually considered safe. Outside the U.S., this unit is usually called the FNU (formazin nephelometric unit).
The invention focuses on the in-line measurement turbidity of the water before and after processing in the water cleaning or water treatment facilities. There are two standard specifications for turbidity measurement that are generally in use worldwide. These are the International Standard ISO 7027 (Water quality—Determination of Turbidity, International Standard, Third Edition, 1999-12-15) and the USEPA 180.1 (Nephelometric Method 2130 B, Standard Methods for the Examination of Water and Wastewater, 1989). Both methods measure the intensity of light scattered at 90° to the path of incident light. The specification of the ISO standard is more stringent and requires the use of a monochromatic light source. This is a need for a greater reproducibility of measured values and for a greater agreement between existing measuring instruments, such as those provide by OMEGA Engineering, Inc. (Stamford, Conn., USA).
U.S. Pat. No. 6,324,900 describes a turbidity sensor with the capability of cleaning the interface surfaces immersed in water. This sensor operates by measuring the light that is scattered under a 90 degrees angle. The sensor has focusing converging lenses placed at a distance from the interface pieces. Such an optical design produces a divergent beam at the analytical area. It also describes another embodiment with the interface pieces made of optical fibers. Both embodiments are fail to provide the convergence of the optical beam as specified in the international standard ISO 7027.
The turbidity sensors described in U.S. Pat. No. 5,350,922 and U.S. Pat. No. 4,841,157 use LED light sources to operate with a broad range of scattering angles rather than at 90°, and their optical designs are significantly different from the requirements of the standard ISO 7027.
U.S. Pat. App. Pub. No. 2003/0214653 describes a turbidimeter having an arrangement of internal surfaces, optical surfaces, and optical restrictions to the field of view of both the illumination and the detector means to improve the lower detection limit of the turbidimeter by reducing the detected signal due to stray light. However, it has a pair of parallel channels, rather than two angled channels with a respective tiled axis with a window in parallel with the bottom of the sensor. Also, its light traps do not serve as a fluid inlet. In addition, it has only one photodiode to cover one testing range. Moreover, it does not comply with EPA 180.1 as it does not use any tungsten lamp as a light source. It also does not comply with ISO 7027 as the angle between the excitation beam and the measured scattered light is not in the range 90±2.5° as specified in the ISO 7027.